Recently, there has been an increasing interest in energy storage technology. As the application fields of energy storage technologies have been extended to cellular phones, camcorders, notebook computers, PCs and electric cars, the demand for high energy density of batteries as a power source has been increasing. Lithium secondary batteries have been proposed as a battery that can satisfy such a demand, and their researches are being actively made.
However, the lithium secondary batteries may cause safety problems such as ignition and explosion and are difficult to be produced, because an organic electrolytic solution is used therein. Particularly, the lithium secondary batteries have recently been used under various conditions and environments as their application range is greatly expanded. As a result, a demand for lithium secondary batteries with a higher capacity is gradually increasing. In order to provide lithium secondary batteries with a higher capacity, the operation ranges of an electrode tend to be expanded, for example, into a high voltage. Such a high voltage is favorable in terms of battery capacity, but may cause more serious safety problems.
Generally, a lithium secondary battery is prepared by carrying out an activation process that initially charges a battery in the state of discharging. During such an activation process, a gas may be generated in the battery due to the formation of a passive film on an anode surface and the decomposition of moisture present in the battery. The generated gas may be remained within the battery to result in Li plating, which adversely affect the life time of the battery. Therefore, it is necessary to conduct a degassing procedure during or after the activation process.
In particular, a lithium-containing compound with a layered structure, represented by the following formula (I), has a specific uniform potential in the region of 4.3 to 4.8 V, unlike other cathode materials that have been conventionally known, and should go through an activation process at high voltage conditions above such uniform potential region so as for the compound to exhibit a high capacity through the structural variation thereof. In the activation process, the lithium-containing compound used as a cathode active material is subject to structural variation at a high voltage, from which large amounts of gases may be generated and remained within the battery to deteriorate the transfer of lithium ions and result in Li plating locally. Therefore, gases generated during the activation process should be removed.
However, there has been no technology being effective to remove gases generated during a high voltage activation process for a lithium secondary battery that has a cathode comprising a cathode active material represented by the following formula (I):Li(LixMy−y′M′y′)O2−zAz  (I)
wherein, x, y, y′, and z satisfy 0<x<0.5, 0.6<y<1.1, 0≦y′<0.2, and 0≦z<0.2,
M is any one selected from the group consisting of Mn, Ni, Co, Fe, Cr, V, Cu, Zn, and Ti,
M′ is any one selected from the group consisting of Al, Mg and B; and
A is any one selected from the group consisting of F, S and N.